1. Field of the Invention
This invention relates to a method of maintaining edge strength of a piece of glass during handling, processing and/or shipping.
2. Description of the Prior Art
In the manufacture of architectural glass panels or multiple glazed units, it is critical that the edges of the pieces of glass exhibit adequate edge strength e.g., the edge of the piece of glass preferably should have a modulus of rupture (MOR) greater than 2,700 pounds per square inch (psi). This is particularly true for coated glass used as solar-absorbing glass because the difference in temperature between the center of the glass and the edge of the glass can generate tensile stresses at the edge of the glass of up to about 2,700 psi.
In this regard, consider the following. The center of the solar-absorbing glass absorbs heat from the sun and expands. The edges of the glass which are in the mounting frame are not exposed to the sun and therefore have a lower temperature. In certain instances, the temperature of the glass at the center may be as much as 45.degree. to 50.degree.F. greater than at the edges of the glass for certain times of the day, e.g., in the morning. When glass is heated, it expands. Since the glass is hotter at the center than at the edges, the glass expands more at the center than at the edges. It has been found that edges of glass having an MOR of greater than 2,700 psi have a higher probability of withstanding the tensile stresses acting at the edges of the glass than edges of glass that have an MOR of less than 2,700 psi.
The prior art method of manufacturing solar-absorbing glass generally includes the steps of cutting a piece of glass to a desired size, coating the piece of glass with a transparent metal coating and assembling the piece of glass into a unit, e.g., a multiple glazed unit.
The prior art step of cutting normally includes scoring a sheet of glass, removing trim from the sheet of glass and when necessary seaming the edges of the piece of glass to remove scoring damage. As can be appreciated, scoring damage e.g., wing and/or spall, reduces the edge strength of the piece of glass. This is because wing and/or spall are localized areas of high stress and are susceptible to generating a crack when put under tension.
The necessity of the seaming step and amount of seaming required generally depends on the type of scoring employed. For example (1) surface scoring i.e., a substantially perpendicular discontinuity in a sheet of glass that extends or connects to a major surface of the sheet of glass and is bordered at the major surfaces by wing and/or spall requires the seaming step to remove the edge damage caused by wing and/or spall; (2) subsurface cracking or subsurface scoring i.e., an essentially spall- or wing-free discontinuity or fissure, such as an intentially induced crack or score respectively that is within the thickness of a sheet of glass and extends substantially perpendicular to the major surfaces of the glass but does not extend or connect to either of the major surfaces, requires less seaming, if any, because of the absence of spall and/or wing; and (3) pseudosubsurface score i.e., an intentionally induced discontinuity or fissure in a sheet of glass that extends substantially perpendicular to the major surfaces of the glass but does not appear itself or connect to either major surfaces of the glass. The pseudosubsurface score is accompanied by a zone of damaged glass which is essentially spall- or wing-free, which zone is disposed between or extends between the tip end of the discontinuity and a major surface of the glass. Pseudosubsurface scoring requires less seaming, if any, than surface scoring.
In each instance, the removal of edge damage induced by scoring increases the edge strength of the piece of glass. However, it has been found, that the edge strength of the piece of glass can have an MOR reduction of up to 1,000 psi after the cutting step and prior to using the piece of glass in fabrication of a unit. This reduction in edge strength generally reduces the edge strength of the piece of glass below an MOR of 2,700 psi thereby making the piece of glass susceptible to cracking.
This reduction of edge strength is attributed to unintentional damage e.g., chips and/or scratches, to the edges of the piece of glass which unintentional damage occurs during handling, processing and/or storing of the piece of glass. The edges of the piece of glass can be damaged for example (1) when the piece of glass is loaded on or removed from a coating apparatus or (2) by chips of glass or abrasive particles on rubber pads which support the glass for shipping or storing. The chips or scratches act as localized areas of stress and the piece of glass cracks by the mechanism previously discussed.
To prevent unintentional damage to the edges of the piece of glass, it is possible to handle the pieces of glass with extreme care. However, this does not prevent accidents from occurring which can cause edge damage to the pieces of glass. Further, increased care in handling tends to increase the cost of the finished unit.